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Performance Evaluation of Multipurpose Solar Heating System

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Performance Evaluation of Multipurpose Solar Heating System Mechanics and Mechanical Engineering Vol. 20, No. 4 (2016) 359{370 ⃝c Lodz University of Technology Performance Evaluation of Multipurpose Solar Heating System R. Venkatesh Department of Mechanical Engineering Srinivasan Engineering College Perambalur, Tamilnadu, India V. Vijayan Department of Mechanical Engineering K. Ramakrishnan College of Technology Samayapuram, Trichy, Tamilnadu, India Received (29 May 2016) Revised (26 July 2016) Accepted (25 September 2016) In order to increase the heat transfer and thermal performance of solar collectors, a multipurpose solar collector is designed and investigated experimentally by combining the solar water collector and solar air collector. In this design, the storage tank of the conventional solar water collector is modified as riser tubes and header and is fitted in the bottom of the solar air heater. This paper presents the study of fluid flow and heat transfer in a multipurpose solar air heater by using Computational Fluid Dynamics (CFD) which reduces time and cost. The result reveals that in the multipurpose solar air heater at load condition, for flow rate of 0.0176 m3/s m2, the maximum average thermal efficiency was 73.06% for summer and 67.15 % for winter season. In multipurpose solar air heating system, the simulated results are compared to experimental values and the deviation falls within ± 11.61% for summer season and ± 10.64% for winter season. It proves that the simulated (CFD) results falls within the acceptable limits. Keywords: solar water collector, solar air collector, fluid flow, heat transfer and Compu- tational fluid dynamics. 1. Introduction Solar energy plays an important role in low{temperature thermal applications, since it replaces a considerable amount of conventional fuel. Flat plate collectors are, therefore, the best candidates for solar heating system. The total system efficiency is based on the performance of flat plate collectors. Hence most of the research work so far has been focused on the performance improvement of collector area K.S.Ong (1995). 360 Venkatesh, R. and Vijayan, V. The absorber plate efficiency, selective coatings, various design of absorber plate, thermal losses and insulation, effect of tilt angle, various working fluids and char- acteristics of thermosyphon system which have been analyzed by many researchers Whiller A et al (1965). To enhance the performance of solar air heaters, efforts have been made to reduce top heat losses from absorber, increase heat transfer coefficient and increase contact area between the absorber plate and the air stream. In order to achieve this, different modifications have been suggested by many researchers R.S. Gill et al (2012). A computational analysis of heat transfer augmentation and flow characteristic due to rib roughness over the absorber plate of solar air heaters were presented by Chaube et al (2006). Sahu & Bhagoria (2005) investigated experimentally the heat transfer coefficient by using 90˚ broken transverse ribs on the absorber plate of a solar air heater. They concluded that the roughened absorber plates increase the heat transfer coefficient 1.25{1.4 times as compared to smooth rectangular duct under similar operating conditions at higher Reynolds number. The heat transfer and friction characteristics of rectangular solar air heater duct using rib-grooved artificial roughness was studied by Jaurker et al (2006). They inferred that as comparison to the smooth duct, the presence of rib-grooved artificial roughness yields Nusselt number up to 2.7 times while the friction factor rises up to 3.6 times in the range of the parameters investigated.The performance of solar air heaters having v-down discrete rib roughness on the absorber plate was investigated by Karwa & Chauhan (2010). Analysis of fluid flow and heat transfer in a rib grit roughened surface solar air heater was presented by Karmare & Tikekar (2010). The previous studies on rib roughness over the absorber plate of the solar air heaters indicated that the artificial roughness results in the desirable increase in the heat transfer rate with the penalty of the undesirable increase in the pressure drop due to the increased friction. This paper presents the study of fluid flow and heat transfer in a multipur- pose solar air heater by using Computational Fluid Dynamics (CFD).In the present work, multipurpose solar collector is designed by adding the solar water heater and the solar air heater. The storage tank of the conventional solar water collector is modified as riser tubes and header. It is fitted in the bottom of the solar air heater as an absorber in the normal air heater. Heat energy stored in water from the solar water heater is being sent into the solar air heater. The heat energy absorbed in the air heater is also added into the previous heated water to increase the energy content of the system. 2. Theory Computational fluid dynamics (CFD) is concerned with the efficient numerical so- lution of the partial differential equations that describe fluid dynamics. Dynamics of fluids are governed by coupled nonlinear partial differential equations, which are derived from the basic physical laws of conservation of mass, momentum and energy. In general, most of the engineering problems are governed by non{linear partial differential equations for which the analytical solutions are possible only if partial differential equations are converted into linear form. Hence, Anderson et al Performance Evaluation of Multipurpose Solar Heating System 361 (1984) obtaining the solutions of partial differential equations by analyzed numerical methods. CFD techniques are used in many areas of engineering where fluid behavior is the main element. Numerical analysis is applied to fluid flow and heat transfer problems. All CFD codes contain three main elements: 1. A pre{processor which is used to input the problem geometry, generate the grid and define the flow parameters and the boundary conditions to the code. 2. A flow solver which is used to solve the governing equations of the flow subjects to the conditions provided. There are four different methods used as a flow solver: (a) Finite difference method, (b) Finite element method, (c) Finite volume method, (d) The spectral method. 3. A post{processor is used to display the data and show the results in graphical way in order to be easy to read format. In all these approaches, the following basic procedure is followed • The geometry of the problem is defined. • The space occupied by the fluid is divided into discrete cells, known as mesh. • Boundary conditions are defined. This involves specifying the fluid behavior and properties at the boundaries of the problem. • The equations are solved iteratively. • Analysis or visualization of the solution. 2.1. Governing equations The governing equations of fluid flow represent mathematical statements of conser- vation laws of physics. The following equations are solved by Versteeg & Malalasek- era (1995) and Ferziger & Peric (2002), which subjects to the boundary conditions of respective problems to get the solution of that problem. 1. Continuity equation: @ (ρm ) + O(ρUm ) = −O:(j ) + S (1) @t k k k k 2. Momentum equation: @ (ρU) + O:(ρUU) = −O.π + ρg + F (2) @t 362 Venkatesh, R. and Vijayan, V. 3. Energy equation: " # @ Dp X (ρh) + O:(ρUh) = −O:(q) + (τ : OU)O h j + S (3) @t Dt k k h k 3. Experimentation In a multipurpose solar heating system, solar water heater and air heater are com- bined together, and act as a multipurpose solar air heater by closing the valve V1 &V4. Initially, the valve V1 is opened and V4 is closed to fill the riser tubes of both water and air heater with cold water every morning before commencement of the experiment. The period of testing for each run was between 08:30 am and 04:30 pm. No water was withdrawn from the storage during the experiment. Experiments were conducted for several days of summer and winter. The heat energy is gained by the water heater which is stored in the riser tubes and is transferred to the air available in the air heater. The air available in the air heater will absorb the heat from the solar radiation that directly falls on it and is reheated by the energy stored in the riser tubes. The experiment was conducted at no load and with load on different days in order to analyze the thermal performance and to compare the performance of this MPSAH with that of the conventional solar air heater. The characteristics of this system are described below. 4. CFD modelling A three{dimensional numerical model was developed using the CFD numerical pack- age, FLUENT which is based on the control volume method. An experimental model was used to evaluate the flow patterns. Flow visualization was used to inves- tigate the flow structure. Simulation was performed in Cosmos Express commercial CFD software. The inlet parameters from the experimental studies were used as the input data for simulation. The output parameters from the simulation studies were compared with the calculated output parameters for the different set of conditions obtained by changing variable parameters like mass flow rate of air and water, inlet temperature and intensity of solar radiation. 4.1. Computational domain The 3{D computational domain of the solar air heater, with height (H) of 200 mm, width (W) 1035 mm and total length of 1035 mm, used for CFD analysis is in Fig. 1. The generations of the geometrical parameters of multipurpose solar water and air heater for computational analysis are shown in Tab. 1. These are created by using the method of volume splitting with face. This split- ting of volume is necessary in order to create meshes with less number of elemental volumes to enable the defined problem to be solved in solver. After creating the geometry, boundary layer is created in the entrance and exit faces of the riser tube.
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